Armature stator bars in large generators are usually liquid cooled and contain a combination of individually insulated conductors, comprising both solid and hollow strands, that are typically made of copper. The hollow strands are used to transmit liquid coolant, for example, water, as well as electric current through the length of the armature stator bar. A header, which must be fluid tight and capable of conducting electric current, is affixed to each end of each armature stator bar.
Examples of conventional liquid cooling designs are disclosed in U.S. Pat. Nos.:3,693,036; 4,066,203; 4,199,700, 4,380,362; 4,894,575; 5,791,924; 5,875,539, and 5,717,267. The headers presently in service in generators serve as series electrical connections between the stator bars or phase rings of the armature circuit and are also the sealed enclosure for transferring the liquid coolant to and from the stator bars. Conventional headers are normally brazed to both the hollow and solid strands at each end of the stator bars. Thus, the liquid coolant is in direct contact with the brazed joints, which can result in liquid coolant leaks due to braze joint corrosion. Ensuing hydrogen leakage and water damage to armature insulation can result in costly maintenance outages.
Thus, there is a need for an improved header design that reduces the leak problem. More specifically, there is a need for an improved header design that reduces water contact with the main current carrying brazed joints.
There is a further need for a header design that will allow for a more reliable, corrosion-resistant metal fusing process.
It may also be advantageous to have a header design that permits the use of hollow stranding made from materials other than copper that have lower electrical losses, are more corrosion resistant, permit higher fluid flow velocities without erosion, and which are more resistant to mechanical fatigue.
This invention is a new and improved header design for a fluid tight, current carrying header that reduces direct contact between the liquid coolant and a brazed joint. More specifically, the present invention comprises a stator bar header comprising hollow strands for carrying a coolant, and solid strands, the hollow strands being longer than the solid strands and grouped together to form a hollow strand package that is fused to the outer chamber of the header. This construction avoids water contact with the current carrying braze and groups the fluid carrying hollow strands, in a separate part of the header where a reliable, corrosion-resistant metal fusing process can be applied to provide a reliable leak proof seal.
Further, by separating the hollow and solid strands, hollow strands can be made from materials (other than copper) which have low electrical loss, are corrosion-resistant, permit enhanced fluid flow velocities without erosion, and which are resistant to mechanical fatigue. Since the present invention provides a construction that allows the hollow strands to be made from materials that are stronger than copper, the hollow strands in the present invention can have thinner walls to provide a combination of increased fluid volume and more room for solid strands in the stator bars.
The header of the present invention provides (1) a means of electrically connecting the stator bars of the winding in series and connecting the terminal bars of the winding to connection rings, (2) a sealed pathway for distributing the coolant into and from the bars, (3) segregation of hollow strands into a separate chamber within the header body, and (4) an opportunity to use metals other than copper and that provide long term reliability, are resistant to corrosion, have low electrical loss, are resistant to stress fracture, and permit enhanced fluid flow velocities. The end result is a fluid seal with long-term reliability, and permits an option of choosing a hollow strand material that has enhanced reliability and electrical performance than copper in conventional designs. The present invention can be used in any new liquid cooled armature design or may be retrofitted in machines currently in operation. The present invention is adaptable for use with solid copper connections or directly fluid cooled copper connections.